1. Field of the Invention
The present invention relates to a tyre for vehicle wheels. In particular, the present invention relates to a tyre for vehicle wheels provided with a reinforced bead structure.
Preferably, the present invention pertains to high-performance tyres such as tyres designed for high-power cars or, more generally, tyres intended for high-speed applications and/or demanding driving conditions.
2. Description of the Related Art
A tyre for vehicle wheels generally comprises a carcass structure including at least one carcass ply having respectively opposite end flaps in engagement with respective annular anchoring structures integrated into the regions usually identified with the name of “beads”.
Associated with the carcass structure is a belt structure comprising one or more belt layers, disposed in radial superposed relationship relative to each other and to the carcass ply and having reinforcing textile or metallic cords with a crossed orientation and/or substantially parallel to the circumferential extension direction of the tyre. Applied to the belt structure, at a radially external position, is a tread band, which too is made of elastomeric material like the other semifinished products constituting the tyre.
To the aims of the present description, it is to be pointed out that by the term “elastomeric material” it is intended a compound comprising at least one elastomeric polymer and at least one reinforcing filler. Preferably, this compound further comprises additives such as cross-linking agents and/or plasticizers, for example. Due to the presence of the cross-linking agents, this material can be cross-linked through heating, so as to form the final product.
Respective sidewalls of elastomeric material are also applied to the side surfaces of the carcass structure, each sidewall extending from one of the side edges of the tread band until close to the respective annular anchoring structure to the beads. In tyres of the tubeless type, a coating layer of airtight material, usually referred to as “liner”, covers the inner tyre surfaces.
Generally, high-performance tyres have a so-called “low section” in which the ratio between the section height, measured in the tyre cross-section along a parallel to the equatorial plane of the tyre itself, between the radially internal point of a bead and the radially external point of the tread band, and the section width defined as the maximum tyre width, is equal to or less than 0.55.
In recent production methods, instead of resorting to the manufacture and assembly of semifinished products following the traditional techniques, each tyre is preferably directly manufactured by application of components in succession onto a toroidal support.
For instance, document WO 01/54927 in the name of the same Applicant discloses a tyre in which formation of a carcass ply takes place by laying a first and a second series of strip-like sections on a toroidal support, each strip-like section comprising longitudinal filament-like elements incorporated into a layer of elastomeric material. The sections of the first series are sequentially laid to some circumferential distance from each other, to form side portions to the end flaps of which primary portions of respective reinforcing structures to the beads are applied, each of said primary portions comprising a first and a second annular insert. The sections of the second series are each interposed in the space defined between two sections of the first series, the respective end flaps overlapping the primary portions of the annular structures. A third annular insert is applied against the end flaps of the sections belonging to the second series to enclose said end portions against the second insert.
JP 2004-217157 depicts a bead structure of a tyre, in which at least two inserts are disposed, each being formed by spirally winding up an elongated element and circumferentially extending it into a bead portion. Said inserts engage a carcass ply, from axially opposite positions for example. The bead structure further has reinforcing elements defined by a composite material made of rubber and fibres. These elements consisting of a non-woven fabric and rubber components, are positioned close to a side surface of the bead inserts formed of elongated elements or close to a side surface of the carcass ply.
US 2002/0170647 shows a tyre comprising at least one reinforcing structure anchored into the bead on both sides of the tyre and circumferentially extending from the bead to the side wall. This document further shows circumferential cords, preferably in radial superposed relationship, forming a group of main cords disposed in each of the beads. Finally a group of secondary cords of the textile type is provided, which cords consist of a zero-degree single cord spirally wound up from the smaller diameter to the larger one.
The Applicant has observed that tyres made following the teachings of the above mentioned documents show a sudden decrease in the rigidity of the sidewall in the radially external region relative to said annular inserts and close to the inserts themselves.
This condition gives rise to a reduced side rigidity of the tyre that adversely affects the obtainable handling in terms of road behaviour.
In fact, the Applicant has observed that said low-rigidity region works like a hinge around which the sidewall rotates when the tyre is submitted to stresses during running. This phenomenon is further emphasised due to the fact that said low-rigidity region is close to the end of the fitting engagement defined by the rim on which the tyre is mounted.
In this respect it has been also observed that this phenomenon is particularly apparent in low-section tyres, in which the reduced section height ensures a sufficient sidewall rigidity by itself and deformation is concentrated on the hinge region as above defined.
The Applicant has observed that the reinforcing elements defined by the rubber and fibre composite material of document JP 2004-217157 and the secondary cords of the textile type of document US 2002/0170647 are not able to ensure the necessary rigidity to the tyre sidewall, above all if this tyre is of the type designed for running at high speeds and/or in the sports field.
In fact, the reinforcing elements of textile material do not offer any resistance to compressive stress and, when submitted to this types of load resulting from stresses to which the tyre is submitted in use, suffer a quick decay that can lead to separation from the blend forming the rubber coat.
The Applicant has perceived that this behaviour gives the tyre a weak fatigue strength.
In addition, the reinforcing elements of the textile type are not thermally very stable and, to the running temperatures, help in causing a decay of the tyre performance, as a macroscopic effect under hard handling conditions.
The Applicant has understood that, above all in high-speed applications and/or in the sports field, there is a need for a tyre, of the type disclosed in WO 01/54927, that is able to withstand high efforts, in particular side efforts due to hard handling conditions.
In particular, the Applicant has felt the need for a tyre provided with a reinforced bead, adapted to ensure the necessary rigidity and structural strength in the above mentioned applications.
More particularly, the Applicant has felt the need to significantly reduce the region of sudden rigidity decrease at the sidewall.
In addition, the Applicant has become aware of the necessity to manufacture a tyre that is able to substantially maintain its performance when used under hard handling conditions and also to ensure a high fatigue strength.
Therefore, the Applicant's efforts have been addressed to modifying the tyre bead regions in order to obtain lateral, longitudinal but also torsional rigidity, combined with the desired fatigue strength.
The Applicant has perceived that through application of reinforcing elements close to the beads, important advantages can be achieved in terms of lateral, longitudinal and torsional rigidity, while obtaining the necessary fatigue strength and thermal stability to the running temperatures.